I'm in the middle of upgrading our home network PCs to Macs and have a few basic questions.

Firstly i'm planning to purchase a imac or mini mac to serve as the main os x server computer. I'm then going to set up network user log ins with open directory so users can then use a macbook air (or any future macs) to log in to the network and access much larger storage drives connected to the imac/mini mac - mainly for itunes and iphoto libraries since that is the MBAs weakness. i'll also be using server for other features but these are the main questions i have first.

(1) Can the imac or mini thats the server, also be used as a client machine and be logged into using a network log in? Or does the server stay on as a server computer that cant be used as a normal client computer if that makes sense? if it cant it would seem a waste to buy an imac for this purpose.

(2) If the MBA is taken outside of the network (and not in any wifi area) can users log in or do they need local machine logins? If so is there any mac os x feature which then syncs the local log in and the network log in?

(3) lastly can the server computer run plex media server or would a user have to be logged into the server machine as a client to be running this?

(1) The only way I log in remotely to work locally in it as a client machine is through sharing screen or ARD (apple remote desktop). They are both extremely slow and not intended for that so I use them just for server administration or specific issues (i.e Managing my home iTunes library which is shared with every other Mac and ATVs). Of course command line works as expected on a Unix system through ssh. So unless you plug a display, keyboard and mouse in, server stays as a server most of the time. That's why the Mac mini is perfect for it.

(2) You definitely can if you configure your network accounts to be mobile. This means that a local copy of your home directory is created on your MBA or any other Mac where you login. So you can take it from your network, log in, work as usual, and then as soon as you get in your network again it will sync with your server automatically in the background. Customizing an account to be mobile is really easy in ML, you just launch users&groups in the systems preference after your first log in on any of the clients and you will see the mobile settings button there. If you want to customize even more, you can download the workgroup manager for ML and customize anything for each user.

(3) No idea, I've never used it. I just have tons of music, photos and movies on it which I process on my Mac Pro and see/listen on my MBP or ATVs

I think you may really only need/want a NAS. I recommend something like ( and use ) Western Digital MyBook Live Duo. They start at 4TB for a little under $400US, if that's not enough to start you out then grab the 8TB. As an aside both can accept a usb stacked drive addon for even more space, if you need it.

1)Your macs will automatically connect and see any shared space, you can configure users login through the web interface.

2)Yes, via a web interface.

3) not sure about plex, but it can run iTunes server and host videos ( only tested locally ).

Why a NAS? It's easy, very fast, you can even turn on time machine to backup your macs continually.

I have a similar network running, using a Mini Server, two MBPs, and old iMac, a couple of Linux workstations and a single Windows XP PC.

Regarding your questions:
(1) You can log in locally and remote to the server. Remote is somehow restrictive, as it is "local with screen sharing", meaning only one login at a time. Depending on your network (I use wired gigabit Ethernet), performance is not an issue. Also on WiFi (Mini wired to Router, Router sending WiFi on 2.4 and 5 GHz) no performance issue.

With Mountain Lion, the server is actually a fully functional workstation plus server extensions. You can do everything you like on the server with no or very little impact on the server functionality.

(2) In my network, all machines have local logins which are also exactly replicated as network logins on the server (with the "local home dir" option). The setup is a bit tricky, but nothing out of the ordinary if you have some Unix experience and are comfortable with the Terminal. It works perfectly in my network. We can take any MBP to any place in the world, but work fine on the server network too. But no, there is no automated synchronization. But there is also not much to synchronize besides home folder content (if you even want this)

Key issues for the setup:
- make sure all users have the same login name and user id on all machines (see advanced option in the user setup pane) It does not matter what your user id is, but make sure it is the SAME on all machines. Don't forget to do a "chown -R <new user id> <home folder>" (example chown -R 555 /Users/foo) after you changed a user id in the advanced screen. You need to be superuser for that (or use sudo)
- On the server, create the same accounts as "network accounts with local home dir" using the Server App. Same ownership treatment as above.

1) Yes, if there exists an account, either local or OD, any user with an existing account can log in unless you explicitly restrict access to users.

2) Yes, Any OD user or MBA local user can log in to the MBA. Local users can log in anytime, OD users can log in as long as it is connected to the same network as the OD server. If you want OD users to be able to log in while outside the network, you must use Portable Home Directories, (otherwise known as Mobile accounts). When the MBA returns to the home network and logs in, the MBA will sync the home directory to the server.

3) Yes, but a user must be logged in to run PMS. PMS is not a background service.

My suggestion:

Using Mac OS X server as a Plex Media Server running Open Directory is a nice way to manage a home network. You'll need to figure out how to attach LOTS of hard drive space for user home directories and media.

1) Users really have no reason to log into Mac OS X server directly. Users only need access to shared drives. If Plex Media Server is installed on the OSX Server, the clients only need Plex Media Player.

2) Portable Home directories is a good way to go if you anticipate OD users needing to log in when the machine is outside the network. The added benefit is that if the MBA dies, all user data is still on the OD Server. You simply need to bind a new machine, log in and home directories begin synchronizing immediately.

3) I suggest creating a user, either OD or local, that has little to no privileges to do anything but run Plex. If you need to connect to the server as an Administrator, use Apple Remote Desktop and set it to connect to virtual display.